Climate change may end up greening the Thar desert

Climate change — the phenomenon of global warming — has many deleterious consequences but, here and there, a tiny benefit may also accrue.

Imagine the Thar desert of Rajasthan turning into a lush green forestland! Climate change could bring about such transformation — though not right away.

Two researchers, PV Rajesh of the Indian Institute of Tropical Meteorology, Pune, and BN Goswami of the Department of Physics, Cotton University, Guwahati, have postulated that the demise of the Thar desert is a distinct possibility due to global warming.

Rajesh and Goswami based their theory on rainfall and climate data, with a focus on what is known as the ‘Indian ocean warm pool’, or IOWP.

The existence of this warm region in the Indian ocean has been known for decades and is a source of monsoons. And now, with global warming, the IOWP is expanding westwards.

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The IOWP itself is rather fascinating. In a vast open ocean, and around the same latitudes, why should one part be warmer than the rest? “The sun does try to warm everywhere,” Goswami told Quantum. However, atmospheric winds cool the ocean surface through evaporation, sometimes forcing colder, deep waters to come up — a phenomenon known as ‘upwelling’. This happens near the Somalia coast in the western Arabian Sea during summers. The waters in the region are cooler, leaving a ‘warm pool’ elsewhere.

At the warm pool’s western boundary, evaporating waters rise. The spin of the earth yanks them diagonally across India. As a result, the north-eastern region gets rainfall for 150 days, while the north-western region gets only 70 days, Rajesh and Goswami say.

With the IOWP expanding westward, the ‘length of the rainy season’ would result in “a 50-100 per cent increase in the mean summer rainfall over the semi-arid northwest of India”, the paper says.

In essence, the scientists contend that the Thar desert could receive good rains and slowly green.

But when will this happen? In an emailed response, Goswami told Quantum: “It is not possible to say whether it will happen in 50 years or in 100 years, but as global warming is likely to continue at least till the end of the century, westward expansion of the Indian monsoon will continue. The length of the monsoon season is expected to increase from about 70 days to about 90 days and annual rainfall to increase from about 45 cm to about 70 cm by that time. The increased rainfall spread over a longer season will help grow taller vegetation.

“However, during the dry winter season, the plants may die unless water is made available through irrigation or uplift of the water table. So, greening could be accelerated if the run-off during the monsoon season could be harvested.”

In the long term, the increased rainfall has the potential to green the desert and significantly increase food productivity in the region. But in the short term, it is necessary to plan for harvesting the excess water to increase groundwater reserves, says Goswami.

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